Portable apparatuses using secondary batteries are recently widely available. So-called lithium-ion (Li-ion) batteries are widely used as the secondary batteries.
Charging characteristics of conventional secondary batteries are deteriorated once the batteries are over-discharged, and even if they are charged again, the batteries cannot be fully charged. This over-discharged state occurs if the output voltage of the secondary battery becomes less than a predetermined value. Therefore, in order to prevent over-discharged state, an over-discharge protection circuit is provided between the secondary battery and a load.
Various kinds of portable apparatuses using a power supply circuit including the secondary battery explained above use the over-discharge protection circuit to prevent over-discharge. However, even though the over-discharge protection circuit is activated, a current may continue to flow from the secondary battery. If the secondary battery is not charged for a long time in such state, the secondary battery may be in over-discharged state.
The present invention has been made in view of the above facts, and an object of the present invention is to prevent the secondary battery from keeping on discharging a current if the voltage of the secondary battery becomes less than a predetermined value.